This invention relates to a trip unit for a circuit breaker and more specifically relates to a microprocessor-based trip unit.
Trip units for circuit breakers are used to automatically operate the circuit breaker under fault current conditions. The time required for the circuit breaker to open will depend on the fault current magnitude and nature. Trip units, for many years, employed a bimetal member which responded in a predetermined manner to relatively low fault current and an electromagnetic trip member which responded to higher fault current magnitudes.
Trip units which employ digital electronic circuits and mircoprocessors to simulate the behavior of bimetal and electromagnetic trip units are also known. Trip units of these types are typically shown, for example, in U.S. Pat. Nos. 4,423,459, dated Dec. 27, 1983, in the name of Frederick A. Stich and Conrad F. Williams; and 4,338,647, dated July 6, 1982, in the name of Wilson et al.
Digital and microprocessor based trip units of the prior art use combinations of analog and digital circuitry. The units, however, do not perform a true RMS load current measurement, and require additional circuits and transformer components to perform ground fault current analysis. Moreover, prior units have limited accuracy over the full current measurement range and do not take accurate account of the prior thermal history of the circuit breaker.
Prior art current sensing systems for electronic trip units employ a current transformer in each phase and in the neutral, if a neutral is used. The current transformer outputs are applied to respective full-wave, bridgeconnected rectifiers. The rectified output of each line is then filtered and applied to a level detection circuit. The measured peak level is then scaled to 0.707 to produce an output which, supposedly, is related to the RMS current of each respective line. A signal related to the measured RMS current is then applied to timing circuits which cause circuit breaker tripping when measured current of a given magnitude exists for given times.
A major problem in prior art electronic trip units is that the RMS current measured is correct only if the current wave shapes measured are perfect sine waves. If the wave shape in non-sinusoidal, the scaling factor is wrong. For example, in circuits having appreciable line capacitance, such as inverter drives, the current wave shape contains substantial distortion from the sinusoidal shape. In circuits containing transformers, a third harmonic component is present in the wave shape, making it non-sinusoidal. Other sources of distortion of the current wave shape from the sinusoidal shape are well known. As a result of this inaccuracy, the circuit breaker may trip falsely and unnecessarily on too low a line current, or the circuit breaker may not trip when it should, because of the inaccurate measurement.
Circuits are known, which correct the above inaccurate measurement. For example, the output of the measuring current transformer can be connected to a resistor and the temperature of the resistor can be measured, to obtain a true RMS reading. Such a system is expensive and responds too slowly to current changes to be useful for electronic trip units.
Commercially available semiconductor chips exist which produce an RMS output. Such chips, however, are too expensive for use in electronic trip units for circuit breakers.
Sampling systems are also known, in which the instantaneous amplitude of a wave is periodically measured. By measuring a sufficient number of samples each cycle and squaring the value of each sample, and then summing and taking the square root of the sum of the squares, the RMS measurement can be obtained, its accuracy increasing with the number of samples taken. The effective number of samples can be increased by phase shifting the beginning of the measuring point for each sample in each subsequent half cycle. Such asynchronous sampling can be used to obtain the RMS value of any periodic wave shape. However, when this technique is applied to electronic trip units, the circuits become complex and expensive, since the measurement must be made separately for each of the phases and the neutral, if a neutral is provided.
As will be seen, the present invention provides a simple and inexpensive circuit for true RMS current measurement.
Another problem with existing electronic trip units is that they require intermediate current transformers for each phase and for the neutral circuit. Typically, these transofrmers are connected in wye, and are needed to produce a ground fault signal. Such current transformers are expensive and occupy considerable volume in the control housing.
As will be seen, the need for such separate intermediate transformers is eliminated by the present invention.
Another problem with prior art microprocessor based systems is that measurement resolution is poor, requiring relatively large microprocessors. By way of example, it is desirable to use an 8-bit microprocessor because of its low cost. If, however, it is desired to be able to discriminate currents from one-fourth full load (for ground fault detection) to ten times full load current, the current range is 1 to 40. If full load current is 600 amperes, the difference between ten times full load current and one-fourth full load current is (6,000-150)=5850 amperes. An 8-bit microprocessor provides 256 unique 8-bit combinations. Thus, the individual steps are: 6,000/256=23.44 amperes. This provides a resolution of about 23.44A/150A=16% here the 150 amperes is 25% of 600 amperes. However, a resolution of about 5% or less is desired. Therefore, a simple 8-bit microprocessor cannot be used, while still having appropriate resolution.
As will be later seen, the invention provides a novel automatic scale adjustment to permit requisite accuracy, while using an 8-bit microprocessor.
A further problem with existing digital circuits is in the means for providing thermal memory. Thus, in a standard circuit breaker trip unit, the history of prior heating of the breaker due to prior interruption operation, or prior heating caused by closing on capacitive loads, and the like, stays with the breaker. In prior art digital systems, the effect of the thermal history of the breaker was lost after the breaker opens. As will be seen, the present invention provides circuit means for retaining thermal memory.
Still another problem in existing trip systems is that the trip winding of the magnetic latch of the circuit breaker has a large number of turns and thus high inductance. Therefore, it is hard to deliver a trip signal rapidly to the coil because the trip circuit with the coil has a large time constant. Accordingly, when the breaker is operated with very high speed current limiting breakers, it is hard to coordinate the performance of the breakers.